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Computational Mechanics 1 Overview 1–1 Chapter 1: OVERVIEW 1–2 TABLE OF CONTENTS Page §1.1. Where this Material Fits 1–3 §1.1.1. Computational Mechanics ............. 1–3 §1.1.2. Statics vs. Dynamics .............. 1–4 §1.1.3. Linear vs. Nonlinear ............... 1–4 §1.1.4. Discretization methods .............. 1–4 §1.1.5.FEMVariants................. 1–5 §1.2. What Does a Finite Element Look Like? 1–5 §1.3. The FEM Analysis Process 1–7 §1.3.1.ThePhysicalFEM............... 1–7 §1.3.2. The Mathematical FEM .............. 1–8 §1.3.3. Synergy of Physical and Mathematical FEM ...... 1–9 §1.4. Interpretations of the Finite Element Method 1–10 §1.4.1.PhysicalInterpretation.............. 1–11 §1.4.2. Mathematical Interpretation ............ 1–11 §1.5. Keeping the Course 1–12 §1.6. *What is Not Covered 1–12 §1.7. *Historical Sketch and Bibliography 1–13 §1.7.1. Who Invented Finite Elements? ........... 1–13 §1.7.2. G1: The Pioneers ............... 1–13 §1.7.3.G2:TheGoldenAge............... 1–14 §1.7.4. G3: Consolidation ............... 1–14 §1.7.5. G4: Back to Basics ............... 1–14 §1.7.6. Recommended Books for Linear FEM ........ 1–15 §1.7.7. Hasta la Vista, Fortran .............. 1–15 §1. References...................... 1–16 §1. Exercises ...................... 1–17 1–2 1–3 §1.1 WHERE THIS MATERIAL FITS This book is an introduction to the analysis of linear elastic structures by the Finite Element Method (FEM). This Chapter presents an overview of where the book fits, and what finite elements are. §1.1. Where this Material Fits The field of Mechanics can be subdivided into three major areas: Theoretical Mechanics Applied (1.1) Computational Theoretical mechanics deals with fundamental laws and principles of mechanics studied for their intrinsic scientific value. Applied mechanics transfers this theoretical knowledge to scientific and engineering applications, especially as regards the construction of mathematical models of physical phenomena. Computational mechanics solves specific problems by simulation through numerical methods implemented on digital computers. Remark 1.1. Paraphrasing an old joke about mathematicians, one may define a computational mechanician as a person who searches for solutions to given problems, an applied mechanician as a person who searches for problems that fit given solutions, and a theoretical mechanician as a person who can prove the existence of problems and solutions. §1.1.1. Computational Mechanics Several branches of computational mechanics can be distinguished according to the physical scale of the focus of attention: Nanomechanics and micromechanics Solids and Structures Computational Mechanics Continuum mechanics Fluids (1.2) Multiphysics Systems Nanomechanics deals with phenomena at the molecular and atomic levels of matter. As such it is closely linked to particle physics and chemistry. Micromechanics looks primarily at the crystallo- graphic and granular levels of matter. Its main technological application is the design and fabrication of materials and microdevices. Continuum mechanics studies bodies at the macroscopic level, using continuum models in which the microstructure is homogenized by phenomenological averages. The two traditional areas of application are solid and fluid mechanics. The former includes structures which, for obvious reasons, are fabricated with solids. Computational solid mechanics takes an applied sciences approach, whereas computational structural mechanics emphasizes technological applications to the analysis and design of structures. Computational fluid mechanics deals with problems that involve the equilibrium and motion of liquid and gases. Well developed subsidiaries are hydrodynamics, aerodynamics, acoustics, atmospheric physics, shock and combustion. 1–3 Chapter 1: OVERVIEW 1–4 Multiphysics is a more recent newcomer. This area is meant to include mechanical systems that transcend the classical boundaries of solid and fluid mechanics, as in interacting fluids and structures. Phase change problems such as ice melting and metal solidification fit into this category, as do the interaction of control, mechanical and electromagnetic systems. Finally, system identifies mechanical objects, whether natural or artificial, that perform a distin- guishable function. Examples of man-made systems are airplanes, buildings, bridges, engines, cars, microchips, radio telescopes, robots, roller skates and garden sprinklers. Biological systems, such as a whale, amoeba, inner ear, or pine tree are included if studied from the viewpoint of biomechanics. Ecological, astronomical and cosmological entities also form systems.1 In the progression of (1.2) the system is the most general concept. A system is studied by decompo- sition: its behavior is that of its components plus the interaction between components. Components are broken down into subcomponents and so on. As this hierarchical breakdown process continues, individual components become simple enough to be treated by individual disciplines, but component interactions get more complex. Consequently there is a tradeoff art in deciding where to stop.2 §1.1.2. Statics vs. Dynamics Continuum mechanics problems may be subdivided according to whether inertial effects are taken into account or not: Statics Continuum mechanics (1.3) Dynamics In dynamics actual time dependence must be explicitly considered, because the calculation of inertial (and/or damping) forces requires derivatives respect to actual time to be taken. Problems in statics may also be time dependent but with inertial forces ignored or neglected. Accord- ingly static problems may be classed into strictly static and quasi-static. For the former time need not be considered explicitly; any historical time-like response-ordering parameter, if one is needed, will do. In quasi-static problems such as foundation settlement, metal creep, rate-dependent plasticity or fatigue cycling, a realistic measure of time is required but inertial forces are still neglected. §1.1.3. Linear vs. Nonlinear A classification of static problems that is particularly relevant to this book is Linear Statics (1.4) Nonlinear Linear static analysis deals with static problems in which the response is linear in the cause-and- effect sense. For example: if the applied forces are doubled, the displacements and internal stresses also double. Problems outside this domain are classified as nonlinear. 1 Except that their function may not be clear to us. “The usual approach of science of constructing a mathematical model cannot answer the questions of why there should be a universe for the model to describe. Why does the universe go to all the bother of existing? Is the unified theory so compelling that it brings about its own existence? Or does it need a creator, and, if so, does he have any other effect on the universe? And who created him?” (Stephen Hawking). 2 Thus in breaking down a car engine for engineering analysis, say, the decomposition does not usually proceed beyond the components you can buy at a parts shop. 1–4 1–5 §1.2 WHAT DOES A FINITE ELEMENT LOOK LIKE? §1.1.4. Discretization methods A final classification of CSM static analysis is based on the discretization method by which the continuum mathematical model is discretized in space, i.e., converted to a discrete model with a finite number of degrees of freedom: Finite Element (FEM) Boundary Element (BEM) Finite Difference (FDM) Spatial discretization method (1.5) Finite Volume (FVM) Spectral Meshfree In CSM linear problems finite element methods currently dominate the scene as regards space discretization.3 Boundary element methods post a strong second choice in specific application areas. For nonlinear problems the dominance of finite element methods is overwhelming. Space finite difference methods in solid and structural mechanics have virtually disappeared from practical use. This statement is not true, however, for fluid mechanics, where finite difference discretization methods are still important. Finite-volume methods, which directly address the dis- cretization of conservation laws, are important in difficult problems of fluid mechanics, for example high-Re gas dynamics. Spectral methods are based on transforms that map space and/or time dimensions to spaces (for example, the frequency domain) where the problem is easier to solve. A recent newcomer to the scene are the meshfree methods. These combine techniques and tools of finite element methods such as variational formulation and interpolation, with finite difference features such as non-local support. §1.1.5. FEM Variants The term Finite Element Method actually identifies a broad spectrum of techniques that share com- mon features outlined in §1.3 and §1.4. Two subclassifications that fit well applications to structural mechanics are4 Displacement Stiffness Equilibrium FEM Formulation FEM Solution Flexibility (1.6) Mixed Mixed (a.k.a. Combined) Hybrid Using the foregoing classification, we can state the topic of this book more precisely: the computa- tional analysis of linear static structural problems by the Finite Element Method. Of the variants listed in (1.6), emphasis is placed on the displacement formulation and stiffness solution. This combination is called the Direct Stiffness Method or DSM. 3 There are finite element discretizations in time, but they are not so widely used as finite differences. 4 The distinction between these subclasses require advanced technical concepts, which cannot be covered in an introductory treatment such as this book. 1–5 Chapter 1: OVERVIEW 1–6 (a) (b) 3 (c) (d) 4 2 4 r 2r sin(π/n) d 5 1 5 i j 2π/n r 6 8 7 Figure 1.1. The “find π” problem treated with FEM concepts: (a) continuum object, (b) a discrete approximation by inscribed regular polygons, (c) disconnected element, (d) generic element. §1.2. What Does a Finite Element Look Like? The subject of this book is FEM. But what is a finite element? The concept will be partly illustrated through a truly ancient problem: find the perimeter L of a circle of diameter d. Since L = π d,this is equivalent to obtaining a numerical value for π. Draw a circle of radius r and diameter d = 2r as in Figure 1.1(a). Inscribe a regular polygon of n sides, where n = 8 in Figure 1.1(b).
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